Patients who have undergone hematopoietic stem cell transplant (HSCT) for the treatment of hematological malignancies often experience cognitive problems due to cancer and/or its treatment. Indeed, even 5 years post- treatment, 40% of HSCT survivors experience persistent cognitive impairments. Despite its deleterious impact on quality of life, cancer-related cognitive impairments (CRCI) are under-diagnosed and under-treated. There is a dearth of research on interventions to treat CRCI. The limited research examining traditional cognitive rehabilitation approaches to ameliorate CRCI in HSCT patients has been disappointing, failing to yield significant benefit. Moreover, traditional cognitive rehabilitation approaches are costly, require the presence of trained clinicians to work one-to-one with patients, and require patients to live in close proximity to providers. Clearly, there is need for the development of a cognitive rehabilitation intervention for HSCT survivors. The proposed study investigates a promising non-pharmacologic intervention for CRCI - systematic light exposure therapy. Research involving neuroimaging has shown that light can modulate cognitive brain function. Light's effect on cognitive functioning involves a non-image forming system that is unrelated to vision, and is, in part, mediated by a retinal photoreceptor system distinct from rods and cones. Results from preliminary research with breast cancer patients undergoing chemotherapy suggest that bright white light (BWL) exposure is associated with an improvement in overall cognitive functioning but dim red light (DRL) exposure is not. The goal of the proposed research is to determine the feasibility and acceptability of BWL as an intervention for CRCI and to determine the preliminary efficacy of BWL on long-term HSCT survivors' cognitive functioning. The potential mediating effects of circadian activity rhythms and sleep on cognitive functioning will also be explored. The approach will be informed by procedures investigators developed for research with breast cancer patients undergoing chemotherapy and for brain injured patients. Sixty HSCT survivors who are 1 to 5 years post-HSCT will be recruited and randomized to either BWL or DRL. Both groups will self-administer the light for 30 minutes each morning for 4 weeks and standardized outcome measures (including cognitive functioning, CAR, and sleep) will be administered before the intervention, during the fourth week of the intervention, and four weeks post-intervention. The study will be the first to investigate BWL vs. a comparison DRL to ameliorate CRCI among HSCT survivors. If effective, the BWL intervention has the potential to have major public health impact as it can be easily administered and, thus, widely disseminated. Overall, this study would provide important information about BWL as a new cognitive rehabilitation approach, giving health care providers and HSCT survivors a much-needed tool to help with CRCI.